To get instanceid for appoitment in post update plugin

if (context.InputParameters.Properties.Contains("Target") &&

context.InputParameters.Properties["Target"] is DynamicEntity)
{

DynamicEntity entity = (DynamicEntity)context.InputParameters.Properties["Target"];
Guid instanceID = ((Key)entity.Properties["activityid"]).Value;

}


for reference

http://www.patrickverbeeten.com/pages/ContentItemPage.aspx?id=12&item=53&p=true

http://nishantrana.wordpress.com/2009/01/21/understanding-inputparameters-and-outputparameters-of-plugin-context-in-crm40/